Clathrate hydrates are safe and low-cost substances having high latent heat (heat of formation) and promising as heat-storage substances of heat-storage materials which can be used in a temperature zone equal to or higher than 0° C. to which water or inorganic salt aqueous solution cannot be applied. Tetrabutylammonium bromide (TBAB) has been known as clathrate hydrates used as the heat-storage substances. PTL 1 describes a heat-storage material in which disodium hydrogen phosphate is added to prevent supercooling at the time of generating a clathrate hydrate such as TBAB.